Improving the strength of paper and board products

ABSTRACT

The present invention concerns a paper or board product with an improved tensile strength comprising cellulose fibres, one or more zirconium carbonate compound(s) and water-insoluble polyvinyl alcohol fibres, and a process for manufacturing such a product, comprising the steps of forming a combined aqueous suspension of cellulose fibres and water-insoluble polyvinyl alcohol fibres, and pressing it into a paper or board product, or pressing layers of water-insoluble polyvinyl alcohol fibres and cellulose fibres into a product having a sandwich-structure, and treating the product before drying with an aqueous solution of a zirconium carbonate compound by impregnating one or more of the surfaces of the product or the complete product with the solution.

FIELD OF THE INVENTION

The present invention concerns a paper or board product comprising cellulose fibres, a zirconium carbonate compound and fibres of water-insoluble polyvinyl alcohol, and a process for manufacturing a paper or board product.

DESCRIPTION OF RELATED ART

The mechanical strength of paper and board is related to the bonding degree of the fibre network. Thus, improvements of the strength properties of paper and board products are generally attempted through increasing said bonding degree. This may be done, for example, by adding to the paper or board pulp an additive that binds the cellulosic fibres to each other.

Zirconium compounds have been used to some extent in improving the strength of paper and board, particularly the wet tensile strength of the paper coatings. This use is based on said zirconium compounds having the ability to react with the OH groups of the cellulose fibres and with the starch, soluble polyvinyl alcohol and other polymers used as binders, thus forming a strong network.

Water-soluble polyvinyl alcohol (PVA) has been used as a binder for some time. Its disadvantages have, however, also been known. Soluble PVA has a high hydrophilic character, resulting in water penetrating the PVA fibres during the manufacturing process of the paper or board, giving a poor water resistance. Attempts have been made in replacing the soluble PVA with insoluble PVA, such as in GB 1 260 028, but finding the right further additives to use in order to obtain the insoluble PVA, a stable cross-linked product and the desired wet strength has been problematic.

Water-insoluble PVA has been widely used in the construction industry for improving the strength and toughness of cement-based constructs, sprayable masses and building boards. Typical examples are the use of the fibres in the cast concrete of earthquake-resistant buildings, in bridges, in sprayable masses of tunnels, as well as in building boards to replace the asbestos fibre now forbidden for health reasons.

The function of the insoluble PVA is based on its significantly high tensile strength and its capability to form a very strong bond with the concrete on curing, using the OH bonds on the surface of the PVA fibre.

It is commonly known that improving the mechanical strength of paper or board products is problematic, since the methods of achieving this improvement generally involves the addition of auxiliary agents causing deterioration of other qualities of the final product. Further, a large concentration of additives may, as such, make the product more brittle by weakening the fibre-to-fibre bonds and the fibre-to-binder bonds. Thus, there is a constant need for new ways of improving the strength of these products.

SUMMARY OF THE INVENTION

It is an aim of the present invention to provide a paper or board product with improved properties compared to the prior art.

Particularly, it is an aim of the present invention to provide a paper or board product with an improved tensile strength compared to the products of the prior art, which product can be manufactured using the existing equipment of paper mills.

These and other objects, together with the advantages thereof over known products and processes, are achieved by the present invention, as hereinafter described and claimed.

The present invention concerns a paper or board product comprising cellulose fibres, as well as a process for manufacturing a cellulose-containing paper or board product.

More specifically, the paper or board product of the present invention is characterized by what is stated in the characterizing part of Claim 1.

Further, the process of the present invention is characterized by what is stated in the characterizing part of Claim 7.

Considerable advantages are obtained by means of the invention. Thus, the present invention provides a paper or board product having improved tensile strength compared to similar known products. For example, using insoluble polyvinyl alcohol fibre as in the present invention, a paper or board product can be obtained, which has a tensile strength that is up to 3-5 times higher than the strength obtained using, for example, polypropylene fibre.

Next, the invention will be described more closely with reference to the attached drawings and a detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing of the network structure formed between the cellulose fibres, the fibres of water-insoluble polyvinyl alcohol and the zirconium carbonate compound according to the present invention, the fibres being shown as lines and dotted lines, and the zirconium carbonate(s) being shown in the form of their molecular structure.

FIG. 2 is a microscopic image of the end of a single fibre torn from a paper strip manufactured according to the method of the present invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS OF THE INVENTION

The present invention concerns a paper or board product comprising cellulose fibres, one or more zirconium carbonate compound(s) and water-insoluble polyvinyl alcohol fibres, as well as a process for manufacturing a paper or board product.

The term “paper or board product” includes all fibrous cellulose-containing preformed products, such as paper, board and preformed papermaking pulp.

According to a preferred embodiment of the present invention, the product contains one or more further conventional paper chemicals, selected from coatings, binders, fillers, sizing agents, retention agents or other paper chemicals. The further binder(s) can be, for example, soluble polyvinyl alcohol or starch, which can be any modified or unmodified starch, preferably cationic starch, most preferably starch derived from rice, wheat, potato, corn or cassava. The coating(s) can be any conventionally used coating, such as a polymeric coating. The filler(s) may be a mineral filler, preferably calcium carbonate (e.g. precipitated), kaolin, talc, aluminium oxide, titanium dioxide, a mineral sulphate or a silicate. The retention agent(s) may be any resin or further ionic or polymeric retention agent.

The water-insoluble polyvinyl alcohol (PVA) fibres used in the invention have a fibre length of 2-10 mm, preferably 4-8 mm, most preferably about 5 mm. They function by forming hydrogen bonds with the cellulose fibres through their reactive functional groups, such as their OH groups, thus forming a cross-linked network of cellulose and polyvinyl alcohol fibres. This network is further strengthened by the addition of a zirconium carbonate compound.

Polyvinyl alcohol (PVA) may occur in various forms, such as in various stereoisomeric forms. This provides, among others, differences in the solubility properties. For the most common form of PVA, here soluble PVA, water is the most commonly used solvent. This form of PVA is, among others, used in water-soluble films. Thus, the term “water-insoluble polyvinyl alcohol” is used here to describe a form of PVA having a significantly poorer solubility than the commonly used soluble PVA.

In the present invention, the zirconium carbonate compound(s) is/are preferably present in the final paper or board product as an aqueous solution of a zirconium carbonate compound impregnated into the product. The zirconium carbonate compound may be ammonium zirconium carbonate (AZC) or potassium zirconium carbonate (KZC). These are anionic inorganic hydroxylated zirconium polymers commonly available in water based solutions and used in paper coatings, paint and ink formulations, metal surface treatments, adhesives and catalysts.

They function as cross-linkers through the reaction of zirconium with, for example, free carboxylic and hydroxylic groups. One of their advantages compared to more traditional (older) zirconium based cross-linkers is that addition levels of 40-70% are normally enough to give the same cross-linking power.

The mechanism of the reaction of the used zirconium carbonates is that, as the impregnation solution dries, the loss of water causes the zirconium carbonate molecule to become reactive towards functional groups, such as hydroxyl and carboxyl groups, and an irreversible covalent bond is formed. The reaction can be made even more effective using elevated temperatures.

Traditional cross-linkers used in the manufacture of paper or board products are generally organic compounds containing at least two functional groups within their structures that react with the hydroxylic groups of the cellulose molecules. The formed cross-links stabilize the fibrous product, giving the product an increased tendency to return to its original shape after being, e.g., bent or deformed in other ways. Paper or board products manufactured from cross-linked fibre generally have an increased thickness, brightness, light scattering efficiency and opacity, as well as a good strength. The water retention properties of a paper or board product are also improved through cross-linking

Optionally, further cross-linkers can be used in addition to the zirconium carbonate compound(s). These may be, e.g., compounds containing carboxylic groups, such as succinic acid, maleic acid or citric acid.

The present invention also concerns a process for manufacturing a paper or board product, such as the product of the present invention, which process comprises the steps of

-   -   forming a combined aqueous suspension of cellulose fibres and         water-insoluble polyvinyl alcohol fibres, and pressing it into a         paper or board product in a conventional way, or pressing layers         of polyvinyl alcohol fibres and cellulose fibres into a product         having a sandwich-structure,     -   treating the product before drying with an aqueous solution of a         zirconium carbonate compound by impregnating one or more of the         surfaces of the product or the complete product with the         solution, and     -   drying the treated product.

The combined aqueous suspension of cellulose fibres and fibres of water-insoluble polyvinyl alcohol can either be formed directly by mixing these fibres into water, or by first forming both an aqueous suspension of the cellulose fibres and an aqueous suspension of the water-insoluble polyvinyl alcohol fibres, and then combining these suspensions and forming them into the combined suspension.

According to a preferred embodiment of the present invention, an essentially homogeneous suspension is formed of the cellulose fibres and the fibres of water-insoluble polyvinyl alcohol through powerful mixing, e.g. using a high-speed rotating blade agitator, and it is pressed into a paper or board product with an essentially uniform thickness, whereafter the fibres are allowed to cross-link.

The optionally used separately prepared aqueous suspension of cellulose fibres preferably has a fibre content of 5-20 g/l, most preferably 10-14 g/l, whereas the optionally used separately prepared aqueous suspension of water-insoluble polyvinyl alcohol fibres preferably has a fibre content of 3-10 g/l, most preferably 5-7 g/l.

The zirconium carbonate molecules used in the present invention are in a stable form. Since they are complex compounds, their concentrations are generally presented as the concentration of zirconium oxide, ZrO₂. In the present invention this concentration is preferably 1-25 weight-%, more preferably 3-15 weight-%, most preferably 3-10 weight-% of ZrO₂.

According to a preferred embodiment of the invention, the solution of zirconium carbonate compound is spread onto both surfaces of the pressed paper or board product in an amount sufficient to impregnate the outermost layer of the surfaces, the solution thus being absorbed into 1-95% of the fibrous material of the product, preferably into 25-90% of the material, most preferably into 50-80% of the material. This is sufficient to cause a substantial increase in the tensile strength of the product, whereas more caution is necessary when thoroughly impregnating the complete product in order to limit the amount of used solution to an amount not weakening the existing fibre-to-fibre bonds of the cellulose fibres and the fibres of water-insoluble polyvinyl alcohol.

According to another preferred embodiment, the sandwich-structure contains 2-10 alternating layers of polyvinyl alcohol fibres and cellulose fibres, preferably 3-8 alternating layers, most preferably 4-5 alternating layers.

According to a particularly preferred embodiment, the sandwich-structure contains 3 layers, whereby the paper or board product contains, as fibre, only cellulose fibres in its centre layer, with both surface layers being formed using a mixture of cellulose fibre and polyvinyl alcohol fibres.

The paper product is preferably dried, either before the treatment using the zirconium carbonate compound or after the treatment or at both of these stages, at an elevated temperature, which temperature may be in the range of 30-120° C., preferably 50-100° C., most preferably 70-100° C.

The following example is not intended to limit the invention, but to describe one of its preferred embodiments.

EXAMPLES Example 1 Carrying Out the Reaction Between the Fibres and Potassium Zirconium Carbonate

Test Materials:

-   -   Cellulose fibres (acid free)     -   Water-insoluble polyvinyl alcohol fibre (King's PVA-fibre         PF-120/fibre length 5 mm/12.0±0.2 cN/dtex)     -   Potassium zirconium carbonate solution (Raisacoat KZ 20: ZrO₂         concentration 20%)

The cellulose fibre was obtained by tearing apart coarse, acid free paper (250 g/m²) using a high-speed rotating blade agitator in warm water (50° C.). The ratio of fibre/water was 6 g/500 ml.

The water-insoluble polyvinyl alcohol fibre (King's PVA-fibre PF-120/fibre length 5 mm/12.0±0.2 cN/dtex) was mixed into water using the same agitator. The ratio of fibre/water was 2 g/300 ml.

The fibre suspensions were combined (800 ml) and mixed into a homogeneous mixture. The obtained suspension (cellulose and insoluble PVA) was filtered through a metal mesh (mesh 120), and the residual water was partially removed using vacuum suction, leaving a moist fibre mass, containing only a small amount of starch used as glue in the paper.

Finally, the moist fibre mass was pressed into uniform thickness in a conventional manner, and dried at 90-100° C.

Test strips were cut from the dried material and some strips were impregnated completely using diluted (30% or 35%) potassium zirconium carbonate solution, KZC (Raisacoat KZ 20: ZrO₂ concentration 20%), the diluted carbonate solution having a final ZrO₂ concentration of 6% or 7%. Excess KZC solution was removed by pressing the strips between the steel plates, and the test strips were dried at 90-100° C.

The tensile strengths of the test strips prepared from the manufactured fibrous product were measured. The results are given below in Table 1.

TABLE 1 Tensile strengths of test strips (N/mm of width of test strip) KZC tensile strength Fibre (%) N/mm Cellulose fibre 0 10.3 6 g cellulose fibre + 0 10.1 2 g PVA fibre 6 g cellulose fibre + 6 21.6 2 g PVA fibre 6 g cellulose fibre + 7 25.6 2 g PVA fibre

The results clearly show that the zirconium carbonate compound reacts with the fibres in the cellulose-based material, thus improving its strength. It was observed that the addition of only PVA fibre had no essential effect on the tensile strength. This is probably due to the PVA fibres having a very round, straight and smooth cross section compared to the cellulose fibres, making them easy to pull out of the cellulose fibre mass, since they simply slide out from between the cellulose fibres.

However, the reaction with the KZC forms an abundant network between the different fibres and the other components (FIG. 1), causing an up to 2.5-fold increase in the tensile strength. This strong binding of the PVA fibres to the structure could also be observed by tearing single fibres from the test strips and observing the microscopic image of the end of the fibre. The image clearly shows the tearing of the fibre itself (FIG. 2).

The tensile strength can be regulated by adjusting the added amount of water-insoluble PVA fibre and the concentration/amount of ammonium or potassium zirconium carbonate.

Example 2 Manufacturing Laboratory Paper Sheets Using Ammonium Zirconium Carbonate

In this example, oriented laboratory sheets were produced in a dynamic sheet former (Formette Dynamic) in accordance with the method of the present invention, i.e. by forming an aqueous suspension of the cellulose fibres and water-insoluble polyvinyl alcohol fibres, and pressing it into the used laboratory paper sheets, by treating the sheets with an aqueous solution of ammonium zirconium carbonate by impregnating the surfaces of the sheets with the solution, and by drying the treated sheets. The sheets were surface sized, and the produced papers were tested for Tensile, Tear and Internal Bonding Strength (see Table 2).

Used Pulp and Chemicals:

-   -   Eucalyptus SA pulp refined to SR 30 with the Voith Labrefiner         -   SEL 0.4 J/m, cons. 4%, SRE 70 kWh/t     -   King's PVA fiber PF-120/5 mm     -   Mixture of 7% (calculated as ZrO₂) Raisacoat AZ 20 ammonium         zirconium carbonate and 1.5% maize starch (food quality) in         water for surface sizing (AZC/Starch).

Sheetforming Conditions:

-   -   Target grammage of base paper 100 g/m²     -   Wire speed 1000 m/min, jet speed 960 m/min (1.6 bar)     -   Pressing 1+2+3 bar     -   Drying at 90° C.

In the surface treatment of the paper, the sample papers were coated (surface sized) with a paint brush, dried in frames in an oven with air recirculation at 80-90° C. for 30 min, and conditioned at 50% relative humidity and 23° C. for 1 week.

The purpose of these tests was to study the impact of small amounts of PVA fiber on paper strengths. An excess of AZC was used in order to ensure the complete bonding of the PVA fibers with the cellulose matrix. This excess is, however, not necessary. The amount of zirconium carbonate can be optimized further, particularly when increasing the scale of the experiments from the laboratory tests into larger scales.

The below results (Table 2) show that the tensile strength was improved in both MD (+47-52%) and CD (+48-54%). In CD the tearing strength improved up to +57% as a function of the PVA-fibre content. A huge improvement of the Scott Bond could be noticed with the addition of PVA-fibres and AZC-starch sizing. The improvement was around 400% both in MD and CD. In MD the tensile stiffness improved up to +29% as a function of the PVA-fibre content.

TABLE 2 Properties of laboratory sheets according to the invention Side and Number PVA PVA PVA direction of tests Ref. 3% 5% 7% Grammage, g/m2 EN ISO 536: 1195 modif. 5 110 145 144 142 Bulk, cm3/g ISO 534: 2005 5 1.37 1.12 1.12 1.23 Tensile strength, kN/m EN ISO 1924-2-1994 MD 10 11.7 17.2 17.7 17.6 Tensile strength, kN/m EN ISO 1924-2-1994 CD 10 4.83 7.38 7.42 7.13 Tensile stiffness, kN/m EN ISO 1924-2-1994 MD 10 1056 1332 1303 1364 Tensile stiffness, kN/m EN ISO 1924-2-1994 CD 10 470 517 504 510 Tearing strength, mN ISO 1974: 1990 MD 10 866 865 932 900 Tearing strength, mN ISO 1974: 1990 CD 10 1025 1378 1471 1605 Internal bonding strength* J/m2 TAPPI T 569 modif. MD 10 408 >1996 >1996 >1996 Internal bonding strength* J/m2 TAPPI T 569 modif. CD 10 376 >1996 >1966 1683 *Modif. Scott, ( Huygen Internal Bond Tester) Test conditions: 50% RH, 23° C. Ref: Cellulose 

1. A paper or board product comprising cellulose fibres, one or more zirconium carbonate compound(s) and water-insoluble polyvinyl alcohol fibres.
 2. The paper or board product of claim 1, wherein the one or more zirconium carbonate compound(s) is/are ammonium zirconium carbonate or potassium zirconium carbonate or a mixture thereof.
 3. The paper or board product of claim 1, wherein the polyvinyl alcohol fibres have a fibre length of 2-10 mm, preferably 4-8 mm, most preferably about 5 mm.
 4. The paper or board product of claim 1, wherein the cellulose fibres and the polyvinyl alcohol fibres are cross-linked.
 5. The paper or board product of claim 1, wherein the zirconium carbonate compound has a concentration of 1-25 weight-%, preferably 3-15 weight-%, most preferably 3-10 weight-%, presented as a concentration of ZrO₂.
 6. The paper or board product of claim 1, wherein the product contains one or more further coating(s), binder(s), filler(s), sizing agent(s), retention agent(s) or other paper chemicals, or a mixture thereof.
 7. A process for manufacturing a paper or board product containing cellulose fibres, comprising: forming a combined aqueous suspension of cellulose fibres and water-insoluble polyvinyl alcohol fibres, and pressing it into a paper or board product, or pressing layers of water-insoluble polyvinyl alcohol fibres and cellulose fibres into a paper or board product having a sandwich-structure, treating the product before drying with an aqueous solution of a zirconium carbonate compound by impregnating one or more of the surfaces of the product or the complete product with the solution, and drying the treated product.
 8. The process of claim 7, comprising first forming an aqueous suspension of the cellulose fibres and an aqueous suspension of water-insoluble polyvinyl alcohol fibres, and then combining these suspensions and thus forming the combined suspension.
 9. The process of claim 7, comprising forming an essentially homogeneous suspension of the cellulose fibres and the fibres of water-insoluble polyvinyl alcohol through powerful mixing, and pressing it into a paper or board product with an essentially uniform thickness, and allowing the fibres to cross-link.
 10. The process of claim 7, comprising forming 2-10 alternating layers of water-insoluble polyvinyl alcohol fibres and cellulose fibres into the sandwich-structure, preferably 3-8 alternating layers, most preferably 4-5 alternating layers.
 11. The process of claim 7, comprising spreading the aqueous solution of the zirconium carbonate compound onto one or more of the surfaces of the product, and allowing it to be absorbed into 1-95% of the fibrous material of the product, preferably into 25-90% of the material, most preferably into 50-80% of the material.
 12. The process of claim 7, comprising drying the pressed product or the treated product at a temperature in the range of 30-120° C., preferably 50-100° C., most preferably 70-100° C., or by drying first the pressed product and then the treated product at a temperature of said range. 